Field effect transistor (FET) power amplifiers can operate in either class AB, class B, or class C and so on. These amplifiers normally require two voltage sources for proper biasing. The drain voltage of the FET is always positive and requires three to twelve volts DC. The gate voltage requires a negative voltage of about 0.5 to 4 volts DC. In any event conventional self biased amplifiers utilizing a single voltage source have been used in low noise and class A applications. The conventional self biased amplifier is not capable of producing appreciable RF power due to the negative feedback effect in the bias caused by the applied RF power. This is of a particular concern for microwave operation or operation for frequencies in the range between 1 GHZ to 2 GHZ or more. As one can ascertain microwave FETs such as gallium arsenide (GaAs) devices are capable of extremely high frequency operation and are relatively low noise devices because only the majority carriers participate in the operation of such devices.
For example, devices as GaAs MESFETs and other such devices have been widely employed in the microwave frequency band. A microwave amplifier usually consists of a cascade of several active devices with interstage and input/output matching networks. The design and operation of such amplifiers in the various classes as indicated above is well known. However, the design of the bias circuits for monolithic ICs (MMICs) amplifiers in microwave technology is as important as the design of the matching networks. A good RF design becomes useless if the amplifier oscillates due to an improper bias network design. The bias circuit determines the device operating point, (power or low noise), amplifier stability particularly at lower microwave frequencies, temperature stability and often gain. Depending on the application for low noise, high gain, and class operation (A, AB, or B) and for efficiency an optimum DC operating bias point exists. Various networks for biasing FET amplifiers are well known and as indicated above normally require at least two sources namely a positive drain source and a negative gate source or alternatively a positive drain source and a positive source electrode voltage. Examples of typical FET biasing circuits for microwave frequencies can be had by reference to a text entitled GaAs Integrated Circuits-Design and Technology edited by Joseph Mum published by McMillan Publishing Company (1988), chapter 4 entitled "Monolithic Microwave Integrated Circuit Design or MMIC Design". Page 251, Figure 4.37 depicts various FET biasing circuits. In any event, as one will understand power amplifier design is considerably more complex than small signal linear amplifier design due to many factors.
In contrast to small signal amplifier design, power amplifiers are generally designed to provide maximum power to a load at high efficiency. The bias point of the device has a major impact on the device output power and efficiency. While class A operation produces the highest power, class AB or more accurately, class B usually results in the highest efficiency.
In any event, conventional self biased amplifiers utilizing a single voltage source when used in low noise and class A applications are not capable of producing RF power as indicated above due the negative feedback effect in the bias caused by the applied RF power.
The dual bias designs (two voltage supplies) require a power up and down sequence whereby the gate bias is first applied for power up and the drain bias must first be removed for power down. This sequence must be followed to prevent degradation of the device or actual destruction of the device due to the resultant high drain currents. Thus the scheme which is utilized for dual bias devices requires extra bias circuitry to assure such synchronization.
It is therefore an object of the present invention to provide a power amplifier utilizing a single bias voltage source which exhibits improved power gain and power operation.